1. Technical Field
The present application relates to the field of mechanical flight control systems.
2. Description of Related Art
Mechanical flight control systems (MFCSs) have been in use for many years for aiding in the control of various types of aircraft. A MFCS typically used in helicopters is a cyclic control system (CCS). A CCS commonly includes a pilot input device, usually a stick controlled by the right hand of a pilot, connected to hydraulic actuators by various mechanical linkages. The hydraulic actuators are often arranged to connect to and cause changes in the physical orientation of a swash plate. Lateral, forward, and aft movement control of the helicopter is primarily controlled by the physical orientation of the swash plate. A CCS is normally designed such that when a pilot displaces a cyclic stick from a centered position, the attached mechanical linkages cause the actuators to adjust the physical orientation of the swash plate such that the helicopter tends to move in the direction of the stick movement.
A CCS is often described as having particular mechanical characteristics. The mechanical characteristic of a CCS are typically summarized as the effective forces perceived by the pilot through the cyclic stick as the pilot manipulates the cyclic stick. The CCS is normally designed to be balanced such that such that without pilot intervention, the cyclic stick centers to a position called “trim position”. When the cyclic stick is centered or at trim position, no lateral, forward, or aft movement of the helicopter occurs due to the CCS. The major contributing forces which combine to establish the mechanical characteristic of a CCS include: (1) a “breakout force” or “return-to-center force” which is a constant force applied toward centering the cyclic stick to trim position despite how far the cyclic stick is displaced and despite at what velocity the cyclic stick is moved, (2) a “gradient force” or “spring force” that also returns the cyclic stick to a centered position but varies with how far the cyclic stick is displaced from trim position such that the farther the cyclic stick is moved, the stronger the force applied toward centering the cyclic stick to trim position, (3) a constant “friction force” that is opposite to the direction of cyclic stick movement, (4) a “damping force” opposite to the direction of cyclic stick movement and which varies with the velocity at which the cyclic stick is moved, and (5) a “hard stop force” which simulates a mechanical limit of travel of the cyclic stick.
The sources of the above described forces vary. Breakout force often emanates from the combination of mechanical balancing of a CCS, the breakout friction force associated with the joints connecting the various mechanical linkages, and the spring preload force associated with the force-gradient cartridges. Gradient force and spring preload both typically primarily emanate from the inclusion of “force-gradient cartridges” situated along a force path between the cyclic stick and the connection to swash plate actuators. Force-gradient cartridges are typically canisters comprising bi-directional spring elements. Hard stop forces are normally forces transmitted to the cyclic stick for purposes of informing the pilot that the CCS is at its control limit for the current directional command.
Automatic flight control systems (AFCSs) are often incorporated into CCSs such that motors or other devices provide mechanical input to the CCS resulting in automated holding of the cyclic stick and/or automated adjustment of the “trim position”. It is common to incorporate a “trim release button” on the cyclic stick which allows the pilot to move the cyclic to any desired position and then release the trim release button to command the AFCS to hold the current cyclic stick position. Often, the “trim position” or “attitude” can be adjusted by moving a four-way thumb switch on the cyclic stick. If a CCS has good mechanical characteristics, it is easy for the pilot to “push through” the cyclic stick position held by the AFCS by applying force to the cyclic stick without disengaging the AFCS.
If the friction forces of a CCS are too high and/or the mechanical leverage offered by the cyclic stick design is too low, significant negative impacts on the mechanical characteristics of the CCS may exists. For example, a cyclic stick offering a lowered mechanical leverage results in higher breakout forces and amplifies CCS mechanical imbalance resulting in poor control harmony. Where frictional forces cannot otherwise be reduced adequately to accommodate the low leverage cyclic stick, force-gradient cartridges fail to provide proper levels of spring force. With low spring force levels, poor cyclic stick centering occurs during manual operation of CCS and the AFCS is prevented from “back-driving” the CCS. While the above described MFCS advancements represent significant developments in MFCS design, considerable shortcomings remain.